The present invention relates to an optical compensation sheet, a liquid crystal display device employing the same, and a liquid crystal composition employed for forming an optical anisotropic layer of the compensation sheet.
A liquid crystal display device (often referred to as xe2x80x9cLCDxe2x80x9d) comprises a liquid crystal cell, a couple of polarizers, and one or two optical compensation sheets (i.e., optical retardation films) provided between the polarizers. The liquid crystal cell comprises liquid crystal molecules, two substrates between which the molecules are placed, and electrode layers for applying voltage to the molecules.
Owing to birefringence of the liquid crystal molecules, an LCD is liable to give an image colored in yellow or yellowish green. This coloring is undesired in either monochromatic or full-color display, and hence an optical compensation sheet is usually provided to LCD. The optical compensation sheet prevents the display from coloring so as to give a clear bright image, and further it often serves for expanding the viewing angle of LCD.
In place of a conventional optical compensation sheet of stretched birefringence film, a new kind of compensation sheet has been proposed and practically employed. In this sheet, an optical anisotropic layer comprising a liquid crystal compound (e.g., a discotic liquid crystal compound) is provided on a transparent support. The optical anisotropic layer is formed, for instance, by aligning (or orienting) the discotic liquid crystal compound molecules and fixing the alignment on the support. Since the discotic liquid crystal compound generally has a large refractive index of birefringence and shows various alignment forms, it can provide an optical compensation sheet having characteristics which are not given by the conventional stretched birefringence film. The compensation sheet employing a discotic liquid crystal compound is described in, for example, Japanese Patent Provisional Publication No. 6-214116, U.S. Pat. Nos. 5,583,679 and 5,646,703, and German Patent No. 3,911,620 A1. The discotic liquid crystal compounds are described in, for example, C. Destrad, et al., xe2x80x9cMol. Cryst. Liq. Cryst.xe2x80x9d 71(1981), pp. 111; B. Kohne et al., xe2x80x9cAngew. Chem.xe2x80x9d 96(1984), pp. 70; J. M. Lehn, et al., xe2x80x9cJ. Chem. Soc. Chem. Commun.xe2x80x9d (1985), pp. 1794; and J. Zhang and J. S. Moore, xe2x80x9cJ. Am. Chem. Soc.xe2x80x9d 116(1994), pp. 2655.
The compensation sheet employing a discotic liquid crystal compound can be used for an LCD of STN or BTN type in which rod-shaped liquid crystal molecules are aligned under a twisted orientation. In an LCD of STN type, the rod-shaped liquid crystal molecules are aligned under a twisted orientation at an angle of 180 to 360xc2x0 and work under birefringence mode. Therefore, the discotic liquid crystal compound in the compensation sheet for STN-LCD must be homeotropically aligned, and preferably aligned under a twisted orientation having the same angle but in the opposite direction to the alignment of the rod-shaped liquid crystal molecules of the cell. For providing the twisted orientation, a chiral compound is added to a non-chiral discotic liquid crystal compound, or a discotic liquid crystal compound having an asymmetric carbon alone is used [J. Malthete xe2x80x9cMol. Cryst. Liq. Cryst.xe2x80x9d 64(1981), pp. 233-238].
Various rod-shaped liquid crystal compounds show a twisted orientation (chiral discotic nematic phases) by adding a chiral compound. The pitch of the phase can be adjusted by mixing the chiral compound with a non-chiral discotic liquid crystal compound in a selected amount. The term xe2x80x9cchiral discotic nematic phase (discotic cholesteric phase ND*)xe2x80x9d here means a chiral nematic phase of a discotic liquid crystal, which is described in detail in known literatures [e.g., C. Destrade et al., xe2x80x9cPhys. Lett.xe2x80x9d A(1980), 79A(2-3), pp. 189-192].
With respect to non-chiral discotic liquid crystal compounds, however, the methods for forming twisted orientations by adding chiral compounds are disclosed only in the following publications: combinations of ethynylbenzenes and saccharides [Anton Hauser, xe2x80x9cJ. Master. Chem.xe2x80x9d 7(1997), pp. 2223-2229], ethynylbenzenes and optical active nitrofluorenones [K. Peaefcke, xe2x80x9cLiq. Cryst.xe2x80x9d 16(1994), pp. 53-65], and optical inactive triphenylenes and optical active triphenylenes [C. Destrade et al., xe2x80x9cPhys. Lett.xe2x80x9d A(1980), 79A(2-3), pp. 189-192 and Japanese Patent Provisional Publication No. 9-26572]. In the publicly known technology, no mentions are given for an optically active or inactive triphenylene compounds having a polymerizable group in its side chain.
Japanese Patent Provisional Publicaton No. 8-95030 discloses a method in which a cellulose derivative is added to an optically active triphenylene compound having a polymerizable group, so as to prevent the liquid crystal phase from being repelled in the alignment procedure.
In the process for producing an optical compensation sheet, it appears necessary for the liquid crystal molecules to be statistically aligned and the alignment should be fixed without changing the alignment. It is discovered that although the combination of an optically inactive triphenylene compound and an optically active triphenylene compound gives a twisted orientation of liquid crystal phase, it is very difficult to produce a practically satisfactory compensation sheet from that combination. Accordingly, it has been desired to give an optical compensating sheet which shows a stable liquid crystal phase uniformly aligned under a twisted orientation at room temperature.
An object of the present invention is to provide a liquid crystal composition which forms a stable chiral discotic nematic phase of a twisted orientation.
Another object of the invention is to provide an optical compensation sheet having an optical anisotropic layer.
Further, it is an object of the invention to provide a liquid crystal display device giving an enlarged view angle.
There is provided by the invention an optical compensation sheet having an optical anisotropic layer which comprises a polymer of a non-chiral discotic liquid crystal compound having a polymerizable group and a chiral compound.
In the optical compensation sheet of the invention, the optical anisotropic layer preferably has a chiral discotic nematic phase.
In the optical anisotropic layer, the non-chiral discotic liquid crystal compound is oriented at an angle between 45xc2x0 and 90xc2x0 to a plane of the sheet, and can be perpendicularly oriented to a plane of the sheet. Further, the non-chiral discotic liquid crystal compound is preferably oriented under a twisted orientation at an angle between 90xc2x0 and 360xc2x0.
The chiral compound preferably is a chiral discotic liquid crystal compound having no polymerizable group or a chiral non-discotic liquid crystal compound having no polymerizable group.
In the invention, the polymer of a non-chiral discotic liquid crystal compound and a chiral compound are in a weight ratio between 99.9:0.1 and 55:45. The chiral compound is a chiral non-discotic liquid crystal compound having no polymerizable group which has a molecular weight of 300 to 10,000.
In the invention, the non-chiral discotic liquid crystal compound preferably has a triphenylene skeleton in its chemical formula. Representatives of the preferred non-chiral discotic liquid crystal compound having a triphenylene skeleton are represented by the following formula (1): 
in which R1 stands for a substituent ofxe2x80x94(L1)axe2x80x94Arxe2x80x94(L2)bxe2x80x94Z1 wherein each of L1 and L2 independently is a divalent linkage group, Ar is a phenyl group or a cinnamoyl group, Z1 is a polymerizable group, and each of a and b independently is an integer of 1 to 20.
It is preferred that the optical compensation sheet of the invention further has an orientation layer arranged on a transparent support film, and wherein the non-chiral discotic liquid crystal compound is oriented at an angle between 45xc2x0 and 90xc2x0 to a plane of the orientation layer. Further, the non-chiral discotic liquid crystal compound is oriented under a twisted orientation at an angle between 90xc2x0 and 360xc2x0.
The optical compensation sheet of the invention can be prepared by the steps of:
coating a solution containing the non-chiral discotic liquid crystal compound having a polymerizable group and the chiral compound on the orientation layer of the support;
heating the coated solution so that the non-chiral discotic liquid crystal compound would be aligned at an angle between 45xc2x0 and 90xc2x0 to a plane of the orientation layer and under a twisted orientation at an angle between 90xc2x0 and 360xc2x0; and 
polymerizing the liquid crystal compound keeping the alignment.
The liquid crystal composition which is preferably employed for preparing an optical compensation sheet of the invention comprises a non-chiral discotic liquid crystal compound having a polymerizable group, a chiral discotic compound having no polymerizable group or a chiral non-discotic compound having no polymerizable group, and a polymerization initiator, and has a chiral discotic nematic phase.